This invention relates generally to xerographic copying apparatus, and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner liquefy and cause a splitting action in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting," has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the trademark "Teflon" to which a release agent such as silicone oil is applied, the thickness of the "Teflon" being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based (polydimethylsiloxane) oil which possess a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where "Teflon" constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of abhesive material to a rigid core or substrate such as the solid "Teflon" outer surface or covering of the aforementioned arrangement.
In attempts to improve at least the perceived quality of the image fused or fixed by a heated roll fuser, such rolls have been provided with conformable surfaces comprising silicone rubber or "Viton" (trademark of E. I. DuPont for a series of fluoroelastomers based on the copolymer of vinylidene fluoride and hexafluoropropylene). As in the case of the "Teflon" coated fuser, oil release fluids such as silicone based oils have been applied to the surface of the silicone rubber or "Viton" to both minimize offsetting and to facilitate stripping. See, for example, U.S. Pat. No. 3,964,431. When the fuser system is one which provides for applying silicone oil to silicone rubber or "Viton" a low viscosity silicone oil (i.e. on the order of 100-1000 cs) has most commonly been employed.
Donor roll RAM (release agent management) systems have been used as part of roll fuser apparatus for some time. Such a RAM system is disclosed in U.S. Pat. No. 4,214,549 issued on July 29, 1980 to Moser. This patent illustrates a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising a thermoplastic resin. The apparatus includes an internally heated, fuser roll cooperating with a backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The pressure roll is the softer of the two rolls, therefore, the nip is formed by the harder fuser roll indenting the softer pressure roll. The heated fuser roll is characterized by an outer layer or surface which by way of example is fabricated from a silicon rubber or Viton material to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump from which it is dispensed by means of a metering roll and a donor roll, the former of which contacts the release fluid in the sump and the latter of which contacts the surface of the heated fuser roll.
U.S. Pat. No. 3,716,221 issued on Feb. 13, 1973 to Gorka et al discloses a heat and pressure fuser roll wherein the heated fuser roller includes a fusing roller having a resilient fusing blanket supported on the periphery thereof and heating means to heat the fusing blanket to a temperature sufficient to fuse the particulate material on a copy sheet. A backup roller is urged toward engagement with the deformable fusing blanket to press the receptor sheet carrying the particulate material into contact with the fusing roller. The fuser roller is coated with an off-set preventing liquid which is applied thereto from the backup roller at predetermined intervals during operation of the device. The off-set preventing liquid is applied to the backup roller via a wick, one end of which is immersed in a quantity of the liquid which is contained in a receptacle. The application of the liquid to the backup roller is controlled such that it is applied once every eleventh revolution of the fuser roller. In other words ten copy sheets are passed through the fuser and then the fuser and backup rollers are rotated an eleventh time without a copy sheet passing therebetween at which time the liquid is applied to the fuser roller via the wick and backup roller.
Compared to wicks, a donor roll RAM provides a significant oil uniformity microscopically on a copy and also to copy for long runs. One major disadvantage of a donor roll RAM system is it creates a a major thermal load on the fuser during standby as well as during run. The power going into the RAM system at cold start was determined to be 1000 watts and greater than 200 watts during run. Another disadvantage of the donor roll RAM system is the added UMC (unit manufacturing cost) of the system which in the Moser patent comprises a metering roll, a donor roll and a wiper blade. When a wick is used to apply the liquid to the pressure roller, as in the case of the '221 patent, the liquid is applied as drops or droplets rather than in a thin layer.
Not only is it desirable to have a RAM system that uniformly applies the release agent material to the heated fuser roll without draining heat therefrom, it is likewise desirable to reduce the UMC of RAM systems by minimizing the number of components utilized therefor.